Service Entrance Tray Cable

[bilwei]

Old Thread revived; Well guys, the owner has requested a quote to replace the damaged conductors. Please note that the existing tray cables are (3) conductor 500kcmil cu with a #1 bare equipment ground. Qty of (10) parrallel cables. Currently the #1 bare equipment ground in the Tray Cable is used as the grounded conductor. The #1 bare cu is connected to XO in the transformer and the neutral bus in the 4000 amp Main Switchboard. The Main Switchboard has a 4000 Amp breaker with GFCI protection. The transformer is a 2500 kva 12470 X 480-277 Y connected transformer.

Question: Can the #1 bare conductor in the 500 kcmil tray cable be used as the grounded conductor(neutral)?
I did not do the original installation.

 

[Smart $]

... Question: Can the #1 bare conductor in the 500 kcmil tray cable be used as the grounded conductor(neutral)?

250.24(C)(1) & (C)(2) requires the grounded conductor to be not less than 12.5% the area of the largest phase conductor. 3?500=1500kcmil... x12.5%=187.5kcmil minimum assuming copper.

 

[Cow]

So what caused that hole in pic 3&4?

 

[bilwei]

Our investigation turned up pieces of welding slag and welding rods in various places through out the cable tray in contact with the cable. The reason for the slag and welding rods is because they were constantly repairing the cooling towers above by welding patches to bottom of the cooling towers to stop the water leaking. We suspect that either the welding slag or a welding rod penetrated the outer jacket and conductor insulation. Along with the water leaking and the bare ground in the cable along with a posible conductor insulation leakage started a slow burn that eventually caused a phase to phase fault and and eventual catastophic failure. This cable is on the supply side of the Main Service disconnect. There is no protection other than the primary fuse on primary side of the 12470 volt, 2500 kva transformer.

 

[bilwei]

Clarification; Each 3 conductor 500kcmil cable wg contains A, B & C phase along with a #1 bare wire that is used as a neutral. There are (10) cables. which means there are (10) parallel runs of 500kcmil per each phase. Therefore 10 x 500kcmil = 5000kcmil x 12.5% = 625 kcmil. Am I looking at this correctly? I have not ben able to find anything in the code book that addresses this particular installation. My concern is the ability of the #1 bare to carry any available fault current in a phase to ground fault. This cable is on the supply side of the 4000A Main Service Disconnect. The availble fault current will be in the 65K range. The transformer ahead of these cables is a 12,470 x 480-277 volt Y connected 2500 KVA transformer.

 

[bilwei]

Also reference a previous thread I had posted about this same cable tray in which I questioned other branch circuits in the same cable tray without seperation. I reposted it yesterday. It will give a little more in depth perspective on this installation.

 

[augie47]

I say your concern is valid.
Look at 250.24(C)(2). My opinion is that the grounded conductor would need to be a 1/0.

MC manufacturers have cables to address this issue. The same situation occurs with the EGC when paralleling MC cables.

 

[Smart $]

I say your concern is valid.
Look at 250.24(C)(2). My opinion is that the grounded conductor would need to be a 1/0.

MC manufacturers have cables to address this issue. The same situation occurs with the EGC when paralleling MC cables.

Your answer is based on each cable installed in a separate raceway. Cable tray is not a raceway, and especially not "two or more racways" as required for the part of 250.24 you mention. The #1 is appropriate. Even #2?10 qualifies.

 

[bilwei]

Thanks for the input. "Seek Wisdom, it is wise to do so".

Just a little more info to share. We went to the plant site this morning because I wanted to verify that the #1 bare wire was terminated on XO of the 2500 kva transformer. (Originally, I went on someone elses word that the #1 bare wire was used as the neutral) Well it isn't so. The #1 bare is terminated on the grounding lug on the base of the transformer. The XO had (2) wires (size unknown-looked like 2/0 at the best)jumped from the XO to the Case of the transformer. Which means any phase to ground fault will flow from the #1 bare thru the transformer case to XO. Not good, We will put in our bid to connect the #1 bare directly to XO and Check the Main Service disconnect to make sure the #1 is connected to the neutral bar and verify that the grounding electrode system is intact. What a mess. My competition hasn't even looked according to Plant Maintenance manager.

 

[augie47]

Your answer is based on each cable installed in a separate raceway. Cable tray is not a raceway, and especially not "two or more racways" as required for the part of 250.24 you mention. The #1 is appropriate. Even #2?10 qualifies.

You are correct. My error.
If I read the original post correctly, he has (10) 500s per phase so the 12.5% would be
625,000 kcmil.
In this situation is there a requirement that that consist of ONE condctor or can it be parallel condctors ?
If parallel is allowed, would the 1/0 rule of 310.4 not apply ?

 

[Smart $]

You are correct. My error.
If I read the original post correctly, he has (10) 500s per phase so the 12.5% would be
625,000 kcmil.
In this situation is there a requirement that that consist of ONE condctor or can it be parallel condctors ?
If parallel is allowed, would the 1/0 rule of 310.4 not apply ?

Umm... that would be 625kcmil combined, not 625,000kcmil.

No requirement that I can recall that the EGC be one conductor. In fact, see 310.4(E).

 

[augie47]

No requirement that I can recall that the EGC be one conductor. In fact, see 310.4(E).

I may be on the wrong road altogether (a habit of mine), but in this case are we not addressing the neutral conductor a service rather than an EGC ?
As such, if the total kcmil was equal to the 12-1/2% would he not be compliant provided he is not using a conductor less than 1/0 since they are in parallel ?

 

[Smart $]

I may be on the wrong road altogether (a habit of mine), but in this case are we not addressing the neutral conductor a service rather than an EGC ?
As such, if the total kcmil was equal to the 12-1/2% would he not be compliant provided he is not using a conductor less than 1/0 since they are in parallel ?

If in fact the cable EGC was being used as a grounded neutral conductor, the "parallel neutrals" are not required to be a minimum of 1/0 because they are not "installed in two or more raceways", per 250.24. However, 310.4 "permits" them to be run in parallel only if 1/0 and larger. However, Exception No. 2 "permits" #2 and larger under engineering supervision for existing installations.

Nonetheless, we need elaboration from the OP'er, now that I think about it, because he stated in the OP that the EGC's were connected to X0... which is the secondary neutral, not the primary. That means the primary is connected delta (regardless of whether it is wye-wye or delta-wye configured), and the primary cable EGC's are in fact primary EGC's (i.e. no direct connection to primary neutral on the load end) [...but I am open to correction on the matter].

To add to that, 250.24(C) only requires the grounded conductor brought to the service disconnecting means for systems operating at less than 1000 volts, and this system is well above that threshold.